Torque limiting mechanism



Oct. 26, 1965 D. PEASE 3,213,645

TORQUE LIMITING MECHANISM Filed Sept. 3, 1963 Tan INVENTOR- FIG. 2DONALD 1.. PEASE A TTORNE Y United States Patent 3,213,645 TORQUELIMITING MECHANISM Donald L. Pease, Marion, N.Y., assignor to XeroxCorporation, Rochester, N.Y., a corporation of New York Filed Sept. 3,1963, Ser. No. 306,045 1 Claim. (Cl. 64-30) This invention relates toimprovements in clutch mechanisms. More specifically, the inventionrelates to a clutch mechanism for transmitting substantially uniformtorque.

In the process of xerography, for example, as disclosed in CarlsonPatent 2,297,691, issued October 6, 1942, a Xerographic plate comprisinga layer of photoconductive insulating material on a conductive backingis given a uniform electric charge over its surface and is then exposedto the subject matter to be reproduced, usually by conventionalprojection techniques. This exposure discharges the plate areas inaccordance with the radiation intensity that reaches them and therebycreates an electrostatic latent image on or in the photoconductivelayer. Development of the latent image is effected with anelectrostatically charged, finely divided material, such as anelectroscopic powder, that is brought into surface contact with thephotoconductive layer and is held thereon electrostatically in a patterncorresponding to the electrostatic latent image. Thereafter, thedeveloped xerographic powder image is usually transferred to a supportsurface to which it may be fixed by any suitable means.

The cycle above described is typical for forming xerographicreproductions on a support surface and may be carried out manually, orautomatically, as is done with mechanized equipment which performs thecycle repeatedly. In either case, however, after transfer of a powderimage from a plate to a support surface, there usually remains on theplate a residual of unremoved developing powder usually in imageconfiguration and referred to in the art as a residual powder image.Before the plate can be reused for a subsequent cycle it is necessary toremove the residual image to prevent ghost images from subsequentlyreproducing.

In US. Patent 3,099,856, issued August 6, 1963, to Eichorn et al., thereis disclosed an improved apparatus for cleaning residual powder from axerographic plate. As disclosed in this patent, removal of residualpowder is eflfected by rubbing against the drum surface, to be cleaned aweb of fibrous material such as, for example, paper toweling,cheesecloth, flannel, and cotton fiber impregnated with a resin of ureaformaldehyde.

In the type of apparatus disclosed in the above referenced patent, theweb of material is taken from a supply spool and transported around acleaning roll onto a takeup roll. The take-up roll is driven to permituniform winding of the used web material onto the take-up roll. Theseinexpensive and disposable webs of fibrous material are advanced intopressure and rubbing or Wiping contact with the xerographic plate andare gradually and continually advanced to present a clean surface to theplate whereby complete removal of residual powder from the plate iseffected. It has been found that a web of very thin tissue paper is avery suitable inexpensive material that can be disposed of after use.However, this latter material can be readily torn, as can the lightweight cardboard tube forming the take-up spool on which it wound.Accordingly, it is necessary to drive the shaft carrying the take-upspool in a manner to maintain the web of material at slight tension, butthe torque applied to this shaft must be limited to limit the tension onthe web within limits to prevent tearing of the web or the cardboardtake-up spool.

The principal object of the invention is to improve ice clutchmechanisms for applying a uniform torque to a web take-up shaft.

Another object of the invention is to improve clutch mechanisms fortransmitting a drive torque in which the maximum torque transmitted maybe predetermined.

For a better understanding of the invention as well as other objects andfurther features thereof, reference is had to the following detaileddescription of the invention to be read in connection with theaccompanying drawings, wherein:

FIG. 1 is an enlarged view of the clutch mechanism for driving thetake-up roll of the web cleaner apparatus;

FIG. 2 is a view similar to FIG. 5 but with the assembly rotated 90;and,

FIG. 3 is a vector force diagram of the clutch mechanism.

General Referring now to the subject matter of the invention, there isshown specifically in FIGS. 1 and 2, a preferred embodiment of a slipclutch mechanism constructed in accordance with the invention. As shown,the slip clutch mechanism includes the gear 451 which is journaled onthe shaft SH13 for rotation relative to the shaft. The gear 451, whichis a driven gear, is driven in the direction shown by the arrows inFIGS. 1 and 2, that is, in a counter-clockwise direction, when viewedfrom the left end of the shaft in these figures.

The lefthand face of the gear 451, as seen in FIGS. 5 and 6, contacts athrust bearing 452 which butts against a retaining ring 454 secured in asuitable annular groove formed in the shaft SH13, the thrust bearingbeing restrained from rotating with respect to the shaft by the lug 453thereon engaging the retaining ring.

Both the gear 451 and the thrust bearing 452 are formed of Delrin, atrade name of E. I. duPont de Nemours & Co. for a thermoplastic acetalresin A friction clutch element, in the form of a flanged cam 455, facedwith cork 458 secured thereon as by cement, is normally biased intofrictional contact with the righthand face of gear 451 by means of atorsion spring 461 which loosely encircles the flanged cam 455 and asecond clutch element or flanged cam 465, the latter being secured toshaft SH13 by a pin 471 which extends through this flanged cam and theshaft.

Both of the flanged cams 455 and 465 are provided with cam or rampportions 456 and 466, respectively, which are formed, in the embodimentshown, at an angle of to the horizontal, although any suitable cam anglemay be used to obtain the desired result as indicated in the forcediagram described in detail hereinafter. These flanged cams arepositioned on the shaft with the ramp portions in contact with eachother for a purpose to be described in detail hereinafter.

The torsion spring 461, in the embodiment shown, is formed of a length,equal to two turns plus 102 of a third turn when in the free position,but is put under a predetermined load when the clutch mechanism isassembled on the shaft, in order to load the friction faces together,that is, the cork 458 and the righthand face of gear 451. The torsionspring is wound in the direction such that when the spring is assembled,it is in a wound and loaded condition With opposite ends of the torsionspring hooked into suitable recesses formed in the flanged cams. Thetorsion spring thus assembled, tends to rotate the flange cams withrespect to one another, which in turn causes them to expand axially.Since the flanged cam 465 is pinned to the shaft and the flanged cam 455is captured by the gear 451, thrust bearing 452 and the retaining ring454, a normal force is created between the gear 451 and the cork 458secured to flanged cam 455. These are the friction surfaces previouslydescribed.

During operation, the gear 451 is driven in the direction previouslydescribed, while the shaft SH13 is retarded by the webwhich is'beingheld and driven by fric tional contact. The gear 451 through frictionalcontact with the cork 458 secured to the flanged cam 455 tends to rotatethis flanged cam in such a direction, counterclockwise as viewed fromthe lefthand end of shaft SH13 in FIGS. 1 and 2, to wind up the torsionspring. However, this torsion spring, itself, which has previously beenplaced under a load during assembly, is trying to unwind. In thisarrangement, the flanged cam 455 is acted upon by the torque of thetorsion spring 461 minus the torque transmitted by gear 451 and, throughthe cam action of the ramp portions of the flanged cams, this action isconverted into a normal force. This normal force times the coefficientof friction between the cork and the gear is the actual friction forceon the gear 451 which in turn equals the torque of the gear. With thisarrangement, there is a feedback of forces built into the clutchmechanism which tends to even out changes in the coefficient offriction.

To prevent frictional grab or excessive wear between the cork 458 andthe righthand face of the gear 451, the face of the cork 458 in contactwith the gear is provided with radially formed grooves 459 to permitlubrication of these friction elements. For a better understanding ofthe operation of the slip clutch mechanism, reference is made to thevector diagram of the forces in this mechanism shown in FIG. 3, and thefollowing description:

f is the coeflicient of friction for the surfaces, i.e. the gear 451 andcork 458.

N is the normal force F is the friction force which is equal to fN F isthe spring force or preset torque of the torsion spring 461.

is the cam angle from the horizontal (1) At time zero, gear 451 notbeing driven, P is zero because the gear 451 is not driven.

(2) Applying a torque to gear 451, through gear 431, will cause it toslip when the applied torque force equals the friction force Ff.

(3) At this time, with gear 451 rotating, flanged cam 455 is now forcedagainst flanger cam 465 by a force F which is equal to s' f) Tan 0 Thus,this force is less than at time zero, i.e.,

F Ff Tan 0 5 Tan 0 since F; at time zero equals 0.

(4) If the coeflicient of friction 1 increases due to lack oflubrication or because of dirt between the cork 458 and gear 451 orbecause of other causes, F also increases since F =F Thus, the force Fwhich is equal As the coeflicient of friction increases, for any of thereasons previously given, the driving force D increases, but since F F;)D E F (Tan 0 f D can only increase a small amount because if for anyreason 1 increases to a large value,

s Fr Tan 0 will approach zero.

In operation, if the coeflicient of friction suddenly increases and thedriving force increases, the torsion spring will begin to wind upfurther than itsoriginal load position which will cause flanged cam 455to turn relative to fixed flanged cam 465 against the frictional forcebetween cork 458 and gear 451 to cause flanged cam 455 to move towardflanged cam 465, because of the angle of the ramp. This action willallow further sliding action between the gear 451 and the cork 458 onflanged cam 455 thereby decreasing the friction force between the corkand gear 451 and thereby decreasing the drive torque to shaft SH13.

Thus, as the torque transmitted from gear 451 to shaft SH13 approachesthe preset value of the torque set up in the torsion spring 461 when theclutch mechanism is assembled, equilibrium is reached. With thisarrangement, the torque applied to shaft SH13 should never exceed thetorque preset in the device by torsion spring 461.

With this type of clutch mechanism, the torque applied to the take-upshaft can be limited to prevent tearing of the web or damage to thecardboard take-up roll by the use of a spring of the desired outputtorque.

Another advantage of this clutch mechanism is that no adjustment of themechanism is required due to wear of the parts. As the clutch elementsWear, the torsion spring and the flanged cams will take-up this wearwith a very small loss in delivered torque to the take-up shaft.

The torque applied to the shaft through this clutch arrangement issufiicient to permit the gear 451 to drive the take-up shaft to rewindthe web material onto the take-up roll as it is advanced by the cleaningroll cooperating with the drum surface, while still permitting thisclutch arrangement to slip whereby the desired range of tension on theweb material is maintained. In this manner the web material is advancedonly by the action of the cleaning roll and not by the take-up roll.

While the invention has been described with reference to the structuredisclosed herein, it is not confined to the details set forth, since itis apparent that various modifications can be made to the clutchmechanism. This application is, therefore, intended to cover suchmodifications or changes as may come within the purposes of theimprovements or the scope of the following claim.

What is claimed is:

A torque limiting mechanism including:

a shaft journaled for rotation,

a gear journaled on said shaft for rotation relative to said shaft,

retaining means secured to said shaft adjacent one side of said gear toaxially align said gear in one direction on said shaft,

a drive element,

a driven element,

said drive element and said driven element each ineluding a hollowstepped cylinder having an enlarged portion and a reduced portion, thefree end of the reduced portion being truncated to provide a cam surfaceat one end thereof,

a clutch plate mounted on the end of said drive element opposite saidcam surface,

said drive element being rotatably mounted on said shaft with saidclutch plate in contact with the side of said gear opposite saidretaining means,

5 6 said driven element being secured to said shaft adjacent engagementWith said gear to transmit a predetersaid drive element with the camsurface of said mined torque as determined by the load on said drivenelement in intimate contact with the mating preloaded torsion spring.cam surface of said drive element, and a preloaded torsion springencircling the reduced cylin- 5 References Cited y the Examine! dricalportions of said drive element and said driven UNI STATES PATENTSelement with one end of sald torsion spring bemg 1,256,947 2/18 Stampemsecured to the enlarged portion of said drlve element 1,322,119 11/19Kiwul 64*29 and the opposite end of said torsion spring being 2,151,7243/39 wengel et aL secured to the enlarged portion of said driven ele- 1Oment to normally bias said drive element into driven ROBERT C. RIORDON,Primary Examiner.

